One concept for increasing the capacity of optical storage media is to use holographic data storage. In this case the surface or the whole volume of the holographic storage medium is used for storing information, not just a few layers as for conventional optical storage media. Furthermore, instead of storing single bits, data are stored as data pages. Typically a data page consists of a matrix of light-dark-patterns, which code multiple bits. This allows to achieve increased data rates in addition to the increased storage density. One further advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or by using shift or wavelength multiplexing, etc.
In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams, where one beam, the so-called ‘object beam’, is modulated by a spatial light modulator and carries the information to be recorded in the form of the data pages. The second beam serves as a reference beam. The interference pattern leads to modifications of specific properties of the storage material, which depend on the local intensity of the interference pattern. Reading of a recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recording. This results in the reconstruction of the recorded object beam. According to one holographic storage approach the reconstructed object beam is read in transmission (transmission type holographic storage medium). For this approach an optical system is needed on both sides of the holographic storage medium. A different approach is to read the reconstructed object beam in reflection (reflection type holographic storage medium). In this case only a single optical system is required. For this purpose the rear side of the holographic storage medium is coated with a mirror layer. The reconstructed object beam is reflected by this mirror layer and can be read from the same side as used for recording.
As the possibility of wavelength multiplexing already indicates, holographic storage systems are very sensitive to wavelength shifts. For an optimum readout performance, the reference beam should have the same wavelength during reading as the wavelength used during recording. However, when an holographic storage medium is to be read in a different storage system than the one used for recording, wavelength mismatches are likely to occur. In addition, due to ageing effects, temperature changes or the like, changes of the wavelength are even found in the same holographic storage system.
WO 2005/036538 discloses a holographic storage medium, in which a wavelength address hologram is recorded. The wavelength address hologram includes information about the wavelength used for recording. The wavelength address hologram can be used to determine a wavelength mismatch between a recording reference beam and a reading reference beam. However, no information is given on how to compensate for the mismatch.
To overcome the above problem, WO 97/02563 discloses a holographic storage system, which is capable of reading a hologram with a different wavelength than the wavelength used for recording. This is achieved by adjusting the angle of incidence of the reading reference beam relative to a holographic storage medium.